Of projectors used as display devices, there are some large-scale projectors that are rented for digital cinemas and for other purposes. Such a projector is specified with a usage validity period (contract period) on a contract. This kind of projector is used to perform projection after confirming that the current time is within the usage validity period based on a secure clock.
It is difficult to use a real-time clock incorporated in the projector (which will be referred to, hereinbelow, as built-in clock) as a secure clock because of the following aspects.
There are some built-in RTCs (Real Time Clock) that do not use battery back-up in order to keep the price of the projector as low as possible and/or in order to make the projector maintenance-free. Though there are some projectors that use a high-capacity capacitor called a super capacitor to perform back-up of the built-in RTC, even such a projector using a super capacitor has to be activated as often as once every two weeks. Since, if the projector has not been activated for two weeks or more, the power supply from the super capacitor runs out and the value of the built-in RTC returns to an initialized state, it is difficult to confirm whether the device has been used within the time limit for use.
It is possible to use a technology that allows a value to be retained for 5 years or longer even after the power supply has been depleted. However, this needs management of setting changes of the built-in RTC, specifically, management of the monthly accuracy rate of the built-in RTC, management of the authority to change settings, recording of the security log, management of the limit time to which change is permitted per year and others, which gives rise a cost increase problem.
Due to the above described problems, it is difficult to develop a rental-use projector which has a function that prevents illicit use by checking use of the projector during the contract period based only on the built-in RTC.
For this reason, the practice (thinking) is to use a secure clock that is supplied from an external source to ensure that the built-in RTC is synchronized with the secure clock, according to which (based on which) the operation to confirm that the device is used within the aforementioned time limit is performed after waiting until the synchronized state has been established.
The projector disclosed in Patent Document 1 has a network connecting function and a real-time clock function and acquires the time from an NTP (Network Time Protocol) server via the network, in order to turn the projection of video on and off at the previously registered times.